AD831

REV. B

–8–

Low-Pass Filtering

A simple low-pass filter may be added between the mixer and

the output amplifier by shunting the internal resistive loads (an

equivalent resistance of about 14

Ω with a tolerance of 20%)

with external capacitors; these attenuate the sum component in

a down-conversion application (Figure 20). The corner fre-

quency of this one-pole low-pass filter (f = (2

π RC

be placed about an octave above the difference frequency IF.

Thus, for a 70 MHz IF, a –3 dB frequency of 140 MHz might

–1

≈ 82 pF, the

nearest standard value.

IFN

VP

IFP

AP

GND

VN

RFP

RFN

VN

VP

LON

LOP

VP

GND

BIAS

VN

AD831

Top View

50

Ω

50

Ω

6

7

8

4

5

9

10

11

12

13

AN

VFB

COM

OUT

14

15

16

17

18

1

2

3

19

20

=

1

2

π f R

1

89.7 f

Figure 20. Low-Pass Filtering Using External Capacitors

Using the Output Amplifier

The AD831’s output amplifier converts the mixer core’s dif-

ferential current output into a single-ended voltage and provides

an output as high as

±1 V peak into a 50 Ω load (+10 dBm).

For unity gain operation (Figure 21), the inputs AN and AP

connect to the open-collector outputs of the mixer’s core and

OUT connects to VFB.

IF

OUTPUT

IFN

VP

IFP

AP

GND

VN

RFP

RFN

VN

VP

LON

LOP

VP

GND

BIAS

VN

AD831

Top View

50

Ω

50

Ω

6

7

8

4

5

9

10

11

12

13

AN

VFB

COM

OUT

14

15

16

17

18

1

2

3

19

20

Figure 21. Output Amplifier Connected for Unity Gain

Operation

The mixer has two open-collector outputs (differential cur-

rents) at pins IFN and IFP. These currents may be used to pro-

vide nominal unity RF-to-IF gain by connecting a center-tapped

transformer (1:1 turns ratio) to pins IFN and IFP as shown in

Figure 18.

20

2

Q3

R5

1k

Ω

LIMITING

AMPLIFIER

VP

18mA TYP

IFN

IFP

18mA TYP

5k

Ω

5k

Ω

Q1

Q2

Q5

Q6

Q4

R4

1k

Ω

R1

20

Ω

R2

20

Ω

R3

26

Ω

Q7

36mA TYP

LOP

LON

RFP

RFN

VP

BIAS

VN

LOCAL

OSCILLATOR

INPUT

RF

INPUT

1

10

11

6

7

BIAS

CURRENT

IF OUTPUT

VPOS

MCLT4-1H

Figure 18. Connections for Transformer Coupling to the IF

Output

Programming the Bias Current

Because the AD831’s RF port is a Class-A circuit, the maxi-

mum RF input is proportional to the bias current. This bias cur-

rent may be reduced by connecting a resistor from the BIAS pin

to the positive supply (Figure 19). For normal operation, the

BIAS pin is left unconnected. For lowest power consumption,

the BIAS pin is connected directly to the positive supply. The

range of adjustment is 100 mA for normal operation to

45 mA total current at minimum power consumption.

1.33k

Ω

0.1µF

VPOS

NOTE ADDED

RESISTOR

IFN

VP

IFP

AP

GND

VN

RFP

RFN

VN

VP

LON

LOP

VP

GND

BIAS

VN

AD831

Top View

50

Ω

50

Ω

6

7

8

4

5

9

10

11

12

13

AN

VFB

COM

OUT

14

15

16

17

18

1

2

3

19

20

Figure 19. Programming the Quiescent Current